1. Field of the Invention
The present invention relates to techniques for diagnosing a booster of a braking system for an automotive vehicle.
2. Discussion of the Related Art
JP-A-4-56669 discloses an example of a known technology for diagnosing a vacuum booster of a vehicle braking system, for any abnormality. This known diagnosing technique is based on a fact that the vacuum booster becomes inoperative to perform a normal boosting function when a negative pressure of a negative pressure source to which the vacuum booster is connected has been raised above a predetermined upper limit. According to the known diagnosing technology, a pressure switch is used to either mechanically or electrically detect that the negative pressure of the negative pressure source has exceeded the upper limit.
A study by the assignee of the present application has revealed that when the vacuum booster is abnormal, the input and the output of the booster do not have a predetermined or known nominal relationship, and that this fact can be utilized to effect a diagnosis of the booster for any abnormality.
In the light of the above findings, the assignee of the present application have developed an apparatus for diagnosing a vacuum booster, which is adapted to determine that the vacuum booster is abnormal, if the detected input and output do not meet a predetermined nominal relationship therebetween. This apparatus was not publicly known at the time the present invention was made.
As a result of a study of the developed diagnosing apparatus by the present inventors, the inventors obtained various findings regarding the diagnosis of the vacuum booster.
For instance, the present inventors found a possibility that the actually normal vacuum booster is erroneously diagnosed to be abnormal, due to a delayed operational response of the booster while the booster is in a transient state. It is desirable to reduce this possibility in the diagnosing apparatus.
Various design concepts are available in designing the diagnosing apparatus for the vacuum booster. According to one of the design concepts, the diagnosing apparatus is adapted to indicate that the booster is abnormal, irrespective of whether the booster is in a transient or steady state, if any abnormality of the booster is found. In this case, the diagnosing apparatus does not indicate that the booster is normal. This design concept suffers from a relatively high possibility that the booster is erroneously diagnosed to be abnormal when the booster is in fact normal. It is desirable to prevent such an erroneous diagnosis, and to cancel the erroneous diagnosis if it takes place.
According to another design concept, the diagnosing apparatus is adapted to indicate that the booster is normal, irrespective of whether the booster is in the transient or steady state, if no abnormality of the booster is found. This design concept suffers from a relatively high possibility that the booster is erroneously diagnosed to be normal when the booster is in fact abnormal, that is, a relatively high possibility that the booster is not correctly diagnosed to be abnormal when the booster is in fact abnormal. In this case, it is desirable to prevent a failure to correctly diagnose that the booster is abnormal when the booster is in fact abnormal.
According to a further design concept, the diagnosing apparatus is adapted such that when the operating condition of the booster has changed from the normal condition to an abnormal condition, the apparatus immediately indicate the abnormality of the booster. However, this design concept requires the apparatus to make frequent determinations as to whether the booster has become abnormal, and therefore suffers from a relatively high possibility of unnecessary determinations. It is desirable to eliminate the unnecessary determinations.
The diagnosing apparatus for diagnosing the vacuum booster is generally provided with an alarm indicator for warning the vehicle operator of the presence of an abnormality of the booster. On the other hand, it is considered unnecessary to warn the vehicle operator of the presence of an abnormality of the booster if the braking system is capable of producing a sufficiently large braking force. In this respect, it is desirable that the diagnosis of the booster by the diagnosing apparatus is effected depending upon the necessity for the vehicle operator to pay attention to the operating condition of the braking system.